Optical illumination system

ABSTRACT

An optical illumination system for aerial electronic flash photography which employs one or more illumination modules each of which includes a cylindrical light source and a unique troughlike reflector configuration for providing weighted distribution of emitted light to compensate for attenuation factors such as slant range and lens optical fall-off.

[ Jan. 15, 1974 United States Patent [191 Eilert et al.

FOREIGN PATENTS OR APPLICATIONS ml Ta t n e r. n mew lar- EBB 00899 66669 999 llll UWWU .l 32 9 976 3 096 69400 33 3333 OPTICAL ILLUMINATIONSYSTEM [75] Inventors: Richard L. Eilert, Palatine; John J. Klemenz,Mount Prospect, both of III.

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21 Appl. No.: 780,124

Primary ExaminerRobert P. Greinei' C 11 3 nmk Our .JHd a m e1, 1 y mc mm 3 ;1 e r m OOC T w Cm h h Wow .B mm m C .m m m; ma w lr mo n w 7 Sf aaUAfln 3 .M9 0 11531 WWIOA. 4 4b ,M3 N GH 0/ 0 40 4 24 2/ ,2 m 3 ".m LMHZ I." 0 "n 4 u 2 n .c u "r n u u& L cm C I 3 h U .mF l I] 2 I00 5 55 Ill References Cited UNITED STATES PATENTS light source and a uniquetrough-like reflector configuration for providing weighted distributionof emitted light to compensate for attenuation factors such as slantrange and lens optical fall-off.

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O O O Richard L.Eilerr John J. Klemenz Attorneys PATENTEUJAN I 51974SHEET 5 BF 5 Inventors FIG.

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Richard L.Ei|err John J. Klemenz By 2 W frorneys FIG.

OPTICAL ILLUMINATION SYSTEM This invention relates generally to anoptical illumination system and, more particularly, to an opticalillumination system for aerial electronic flash photography.

Equipment which provides a .so-called electronic photography, continuingefforts are being made to overcome the problems in the use of electronicflash equipment in this field. Thus far, these efforts have been largelydirected toward improving the flash tubes and developing lighter weightpower supplies. However, up to this time very little has beenaccomplished with regard to the development of more efficient techniquesfor distributing the emitted illumination with the exception of theIllumination Optical System disclosed by Walter L. Colterjohn in U.S.Pat. No. 3,251,984 and the Optical Illumination System disclosed byThomas L. Thompson and Theodore J. Schulze in U.S. Pat. No. 3,375,36l.Although the Colterjohn and Thompson et al patents show significantaccomplishment in terms of efficiency and uniformity of distribution,there has remained a considerable need for an optical illuminationsystem which has the capability for providing an efficient, well definedillumination intensity distribution which will compensate in a givenapplication for variations in the slant range from the illuminationsource and for the effects of lens fall-off and which has the capabilityof ready adaptability to formats of differing illumination intensitydistribution requirements, configuration, and size. Moreover, theattainment of these capabilities has been most difficult because of thesevere limitations of space availability for the optical illuminationsystem on aircraft coupled with the use of light sources of increasinggreater energy output and the concomitant problem of cooling, in alimited space, equipment which provides short duration but extremelylarge, repetitive energy output.

Therefore, it is a general object of this invention to provide a new andimproved optical illumination system for use in aerial flashphotography.

It is another general object of this invention to provide an improvedoptical illumination system which makes .more efficient use of theemitted illumination then previous systems.

It is a primay object of this invention to provide an opticalillumination system which distributes illumination intensity withcompensation for variations in the slant ranges from the illuminationsource and for lens optical fall-off.

Another primary object of this invention is to provide an opticalillumination system which is readily adapt able to differing formatconfigurations and size.

It is another object of this invention to provide an opticalillumination system for aircraft which is adaptable to thespace-availability limitations on aircraft.

Still another object of this invention is to prolong the life of theequipment by animproved system of heat dissipation.

These and other objects of the present invention are achieved by theemployment of a novel illumination reflector system. Briefly, theinvention is basedon the discovery that a weighted reflective surfacewhich is uniquely capable of efficiently distributing emittedillumination in a manner which will compensate for variations in slantrange, lens fall-off, and other inequalities can be generated about acylindrical light source in accordance with the recognition that thelight emitted from a cylindrical light source is incident upon a pointon a parallel reflective surface in the form of a set of convergent rayshaving a convergence angle which is dependent upon the radius of thecylindrical light source and distance of the point of incidence from thegeometric axis of the lightsource and that the emitted light isreflected from the point of incidence in the form of a set of divergentrays having a divergence angle which is dependent upon the convergenceangle of the set of incident light rays and a direction which isdependent upon the slope of the reflective surface at the point ofincidence. In particular, the present invention involves the employmentof a reflective surface which is generated to extend through a series ofpoints each located at a distance from the geometric axis of thecylindrical light source and at a slope such that (l) at each of thepoints in the series, the tangentially-emitted incident ray nearer thedesired aperture of the reflector is reflected at the same predeterminedangle with respect to the plane defined by the geometric axis of thelight source and the center line of the desired illumination beam so asto fall upon the opposite, outermost edge of the desired illuminationformat and (2) the divergence angle of the set of reflected rays isgradually reduced at each of the points in the series so as to diminishfrom a predetermined maximum angle at the rearmost point on the surfaceto a predetermined minimum angle at the fo'rwardmost point on thereflective surface. Before proceeding further with a description of theinvention, it may be well to review at this point certain backgroundconsiderations.

The camera normally employed in aerial photography has a square orrectangular format area. The optical illumination system, therefore,should be capable of illuminating a ground format which is coincidentwith the area imaged upon the camera format. Because sufficient lightoutput must be provided to insure adequate film exposure at theoperating altitude (taking into account photographic parameters such aslens relative aperture, film sensitivity, spectral considerations, andscene reflectance characteristics), the ground format must beefficiently illuminated with a minimum of spill of illumination into theregion outside of the ground format. Moreover, the illumination beamshould have an intensity distribution such that proper compensation isobtained for variations in slant range from the illuminator source topoints on the ground form at and for the effects of lens fall-off.Basically, these are the accomplishments of the inventive illuminationsystem.

In a typical system embodiment of the invention, one or moreillumination modules have reflective surfaces generated in the mannerpreviously noted are oriented parallel to the direction of flight, andone or more such reflector units are oriented perpendicular to thedirection of flight. The reflector units oriented parallel to the flightpath provide format illumination weighted to compensate for attenuationdue to slant range and lens fall-off in the port and starboarddirections. The reflector units oriented perpendicular to the flightpath provide format illumination weighted to compensate for attenuationdue to slant range and lens fall-off in the fore and aft directions. Thesystem is readily adaptable to formats of differing illuminationintensity distribution requirements, configurations, and size, and willadapt to limitations on space availability on aircraft and accommodatethe use of cylindrical light sources of increasing greater energyoutput. It has been found that the system can be effectively cooled bymounting the cylindrical light sources within quartz tubes and providinga manifold system which passes ram intake air through the quartz tubes.

In addition to increased format illumination distribution efficiency,significant improvement in system illumination output efficiency hasbeen achieved with the reflectors of the present invention, largely dueto the reflectors increased collection efficiency in a smaller physicalsize and because the reflector utilizes radiation directly from thelight source. In the prior parabolic aerial illumination systemreflector units, the sides of the lamps away from the main parabolicreflector were covered by small reflectors which reflected half of thelamp output back into the parabolic reflector, resulting in double lossof energy due to the reflectivity coefficients of the materials.

These and other features and objects of the present invention will bebetter understood by reference to the following detailed description inconjunction with the accompanying drawings.

In the drawings:

FIG. 1 is a perspective view of an illumination unit according to thepresent invention with certain parts shown in exploded position and witha portion of the reflector shown broken away.

FIG. 2 is a cross-sectional view taken at 22 of FIG.

FIG. 3 is a cross-sectional view taken at 33 of FIG.

FIG. 4 is a cross-sectional view similar to FIG. 2 but in diagrammaticrepresentation.

FIG. 5 is a diagrammatic representation for use in explaining the formof the principal reflective surfaces.

FIG. 6 is an enlarged diagrammatic view of the most rearward portion ofthe reflector for use in explaining a modified form of this portion ofthe reflector.

FIG. 7 is a diagrammatic cross-sectional view taken along the axis ofthe cylindrical illumination source.

FIG. 8 is a perspective view of a typical structural arrangement of anillumination system according to the present invention.

FIG. 9 is a perspective, diagrammatic view of photographic formatillumination to aid in explaining a typical application of the presentinvention.

FIG. 10 is also a perspective, diagrammatic view of format illuminationbut arranged to provide a topological representation of groundirradiance.

FIG. 11 is a partial plan view of an illuminated ground format with theamplitude of ground irradiance indicated at plural points within theformat.

FIGS. 12 through 16 are a series of diagrammatic plan views of thephotographic format to aid in explaining the use of several illuminationunits to provide a composite format illumination in accordance with thepresent invention.

FIG. 17 is a diagrammatic plan view of the photographic format with acomparison by sectors of the theoretical ground irradiance with groundirradiance provided by the illumination system application described.

THE ILLUMINATION MODULE Referring now to the drawings, and moreparticularly to FIGS. 1 through 3 thereof, there is illustrated apreferred form of an illumination unit or module 10 in accordance withthe present invention. In FIG. 1, to promote ease of understanding,certain parts of the illumination module 10 are illustrated in explodedposition, and a portion of the reflector isshown broken away. Theillumination module 10 comprises generally a cylindrical electronicflash tube 11 mounted within a trough-like reflector 12 which isarranged to provide an illumination beam of rectangular cross-sectionalconfiguration. The flash tube 11 is located within, and is coaxial with,an optically transparent quartz tube 13 which extends through respectiveopenings 14 in the end plates 16 and 17 of the reflector. The ends ofthe quartz tube 13 are respectively received and supported within thebores 18 provided in the mounting blocks 19 of a pair of identical endmounting assemblies 20. The mounting blocks 19 are fabricated ofelectrical insulating material and are affixed to the respective endplates 16 and 17 of the reflector by screws 21 or other suitablefastening means. The electrode terminals 15 at the ends of the flashtube 11 are each tightly received and supported by anelectrically-conductive connecting clip 22. The connectingclips 22 areeach secured in place by a compatible slotted recess 23 in thecorresponding mounting block 19 and by an end fitting 24 which, in turn,is affixed to the mounting block 19 by the screws 21. Each of the endfittings 24 is fabricated of electrical insulating material and includesa cylindrical portion 25 providing an air passage to the bore of itsassociated mounting block 19 and the interior of the quartz tube 13.Each cylindrical portion 25 is capable of receiving an intake airmanifold conduit. Thus, the quartz tube 13, the mounting blocks 19, andthe end fittings 24 cooperate to form a cooling duct. Cooling air can bemanifolded to enter either end of this duct and exhaust from theopposite end. To enhance heat dissipation, the connecting clips 22 areeach provided with a plurality of radial fins 26 interposed in thecooling air flow path.

As illustrated in FIGS. 1 and 2, the reflector 12 is preferably providedwith an outwardly-extending flange 31 along the perimeter of itsillumination beam aperture. The purpose of this flange is to enhance thestructural integrity of the reflector 12 and to provide one means formounting the illumination module 10 in a frame.

It should be noted that the typical reflector 12 of the presentinvention illustrated in FIGS. 1 through 3 has a pair of interior,curvilinear, principal reflective surfaces 27 and 28 which aresymmetrical about a median plane 30 defined by geometric axis 29 of thecylindrical light source 11 and the center line of the reflector beamaperture. It is the function of these principal reflective surfaces 27and 28 to provide efficient distribution of emitted light weightedlaterally (i. e., weighted in the directions perpendicular to the medianplane 30). The end plates 16 and 17 are also provided with interiorreflective surfaces which, however, are not capable of distributinglight in a weighted manner as precise as the reflective surfaces 27 and28. Hence, in a typical illumination system according to the presentinvention, one or more illumination modules will be arranged with theaxes of their cylindrical light sources oriented parallel to thedirection of flight to provide weighted distribution of light in theport and starboard directions, and one or more illumination modules 10will be arranged with the axes of their cylindrical light sourcesoriented perpendicularly to the direction of flight to provide weighteddistribution of light in the fore and aft directions. A typicalillumination system arrangement is illustrated in FIG. 8 and will bedescribed further on. At this point it will be helpful to gain a morespecific understanding of the design of the reflective surfaces of theillumination module 10.

Referring now to FIG. 4, a cross-sectional view of the illuminationmodule 10 taken in a plane perpendicular to the axis of the cylindricallight source 11 is presented in somewhat diagrammatic form. Aspreviously indicated, the curved reflective surfaces 27 and 28 functionto provide efficient distribution of emitted light incident ray A nearerthe aperture: of the reflector i2 is reflected at the half beam-width.angel of lS- w degrees with respect to the median plane 30 so as to fallon the opposite, outermost lateral edge of the desired illuminatedformat. The angle of convergence C and the corresponding equal angle ofdivergence D, are determined by the distance of the point P, from thegeometric axis 29 of the illumination source 11 and the radius of theillumination source. Thus, if the point P is at a distance so as toprovide an angle of divergence D equal to 17 degrees, the tangential rayB, from the more rearward side of the illumination source 11 will bereflected at an angle of 1-5: degrees with respect to the median plane30 so as to fall in the opposite half of the illuminated format but inthe central region of the format.

At the more forwardly located point P the slope of the reflectivesurface 27 again is such that the tangentially-emitted incident ray Acloser to the reflector aperture is reflected at the half-beam-widthangle of IS-r degrees relative to the median plane 30 so as to fall onweighted in the directions perpendicular to themedian plane in order tocompensate for slant range and lens optical fall-off in thesedirections. This is accomplished in accordance with the recognition thatthe plasma discharge are of the light source 11 is a cylindrical sourceand not a line source. A high energy gaseous discharge flash tube may,for example, have a diameter of 12 millimeters. Thus, the light emittedfrom the cylindrical light source 11 is incident upon a given point onthe parallel reflective surfaces 27 and 28 in the form of a set ofconvergent rays having an included angle of convergence which isdependent upon the radius of the cylindrical light source 11 and thedistance of the point of incidence from the geometric axis of the lightsource. The light reflected from the point of incidence is in the formof a set of divergent rays having an included angle of divergence whichis equal to the angle of convergence of the incident light and adirection which is dependent upon the slope of the reflective surface atthe point of incidence. In accordance with the present invention, thereflective surfaces 27 and 28 are each formed to pass through a seriesof points each located at a distance from the geometric axis of thecylindrical light source and at a slope such that (l) thetangentially-emitted incident ray nearer the,desired aperture of thereflector at each of the points in the series is reflected at apredetermined half-beam-width angle with respect to the median plane 30so as to fall upon the opposite, outermost lateral edge of the desiredillumination format, and (2) the divergence angle of the set ofreflected rays is gradually reduced at each of the points in the seriesso as to diminish from a predetermined maximum angle at the rearwardmostpoint on the reflective surface to ,a predetermined minimum angle at theforwardmost point on the reflective surface. a

In FIG. 4, two typical points P and P are illustrated on the reflectivesurface 27. Assuming for the purpose of explanation that the desiredbeam width of the illuminated format is 37 degrees, the slope of thereflective surface 27 at point P is such that tangentially-emitted theopposite, outermost edge of. the illuminated format. The point Phowever, is at a greater distance from the axis 29 of the illuminationsource than the point P such that the angles of convergence C anddivergence D are smaller. Thus, if the angle of divergence D: from thepoint P is 6 degrees, the tangential ray B willbe reflected at an angleof l2-5 degrees with respect to the median plane 30 so as to fall inthe'opposite, outer quarter of the illuminated format.

Thus, the points P and P typify the manner in-which the reflected lightfrom the reflective surface 27 is weighted to provide a greater amountof light near the outer edge of the illuminationformat where theattenuation due to slant range and lens fall-off is greatest.

It has been found that by computing the requisite slope and distancefrom the illumination source axis at a sufficiently large yet finitenumber of points, a reflective surface can be generated for a particularweighted light distribution application in the form of substantially acontinuous curve. Since the more points used in generating the surfacethe more accurate will .be the light distributed, the computations arepreferably accomplished by a reiterative computer program. In order tobetter understand the configuration of the reflective surface and thebasis for writing a computer program for a particular application, adescription follows of the diagrammatic representation of FIG. 5.

In FIG. 5, a given point P, on the reflective surface 27 may beexpressed in polar coordinates as being at a distance L from thegeometric axis 29 of the illumination source ll at an angle X withrespect to the desired median plane 30. Considering the illuminationsource 11 to have a radius R, the convergence angle C, of the incidentlight and the divergence angle D,, of the reflected light will each beapproximately equal to 2 (arctan R /L The slope of the reflectivesurface at point P may be expressed in terms of the angle Y, formed byintersection of the median plane 30 with a line normal to the reflectivesurface at point P The reflected tangential ray A, intersects the medianplane 30 at an angle J and the tangential ray 8,, intersects the medianplane 30 at an angle K,,. The angle Y, is equal to J K,, 2X,,/4. Inorder to provide a continuous surface, the point P must be related toits preceding point P, In polar coordinates, the position of point P isadvanced from the point P, 1 by a small angle AX and at a slightlygreater distance AL. Thus,

L, R/arctan [(J, KQ/Z] The slope Y, is computed from the expression:

Y, (J, K, 2X,)/4

For all subsequent points P,,, AX increments are advanced, K, isselected, and L, and Y, are computed, 1,, being a constanthalf-beam-width angle at all points. The angle K, is gradually decreasedat each point at a rate which will provide the desired illuminationweighting. Thus, having selected K, at a particular point P,,, thefollowing computations are necessary:

The principal parameters to be varied in generating a reflector surfacefor a given beam width and weighting factor are the distance L, to thefirst computed point and the change in the angle K, as the computationproceeds. K, can be varied, for example, as a linear function of AX, asan exponential function, or in other ways. Obviously, the greater therate of increase of K,,, the greater the weighting factor of thereflective surface since more light will be directed toward the outerregion of the illuminated format. In some instances, K, may be anegative angle at the more rearward points on the reflective surface. Atsuch points, the ray B, will fall in the nearhalf of the format ratherthan in the opposite half. It should also be noted that while it wouldbe preferable from the standpoint of precise format edge definition toprevent unwanted light spill outside of the desired format by continuingthe reflective surfaces 27 and 28 sufficiently forwardly that no direct,unreflected emitted light'leaves the reflector aperture at a greaterangle relative to the median plane 30 than a half-beam-width angle,space limitations or other such factors in a given application maynecessitate a more rearward termination of the reflective surfaces 27and 28 permitting a certain amount of direct light to spill outside ofthe desired format. The increase in efficiency due to precisedistribution and weighting of the reflected light enabled by the presentinvention, however, normally renders such limited direct lightspill-over rather negligible.

Referring now to FIG. 6, there is illustrated a modification in which aportion 41 of the reflector 12 behind the illumination source 11 iscontoured to direct reflected light in this region around theillumination source 11 to avoid the blanking effect of the plasma arc.The reflective surface 41 is contoured to direct the sets of light raysfrom the several exemplary points M through M in a hugging" ortangential manner around the plasma until the point P, is reached atwhich the generation of the reflective surface 27 is begun in the mannerdescribed above. It should be noted at this juncture that other contoursmay be used for the reflective surface 41 if the blanking effect of theplasma is not considered significant. For example, if the point directlybehind the illumination source 11 on the median plane 30 is at adistance such that the angle of convergence of the emitted light raysincident on this pointis equal to the full beam width angle, this may beused as the location of the beginning point P, for generating thereflective surface 27, or if less than a full beam width is used, thecontour 41 may simply be a suitable contour which will provide a smoothcurvilinear continuity from the reflector surface 28 to the beginningpoint P, of the reflector surface 27.

It should be understood that normally the reflective surface 28 will begenerated as the minor image of the reflector surface 27. In unusualapplications, however, the reflective surfaces 27 and 28 may beasymmetrical or one of these surfaces may be generated in a manner otherthan as described herein.

Referring now to FIG. 7, there is provided a somewhat diagrammaticcross-sectiontaken along the geometric axis of the illumination source11 for the purpose of explaining the design of the reflective surfacesof the end plates 16 and 17 of the illumination unit 10. It has beenfound that the optimum arrangement of reflective surfaces for the endplates is to design these surfaces to look at the large smear orreflected light from the back surface of reflector 12 rather than plasmaarc itself. Even though illumination from the back of the reflector 12is in the form of reflection with intensity reduced by the reflectivitycoefficient, it is a much larger area of light than that of the plasmaarc itself. Thus, in FIG. 1, a pair of facets 42 and 43 are provided inthe end plate 17 and oriented to direct light reflected from the back ofthe reflector 12 to the opposite, outer quarter of the illuminatedformat to provide a form of limited weighting of reflected light.Specifically, the facet 42 is designed to intercept light rays whichleave the back surface of the reflector at a large acute angle relativeto the normal to the back surface and reflect these light rays at anangle so as to fall inthe opposite, outer quarter of the format. Forexample, if the format beam width in a plane perpendicular to the axisof the light source is 74 degrees, the facet 42 may be oriented at anangle of 32 degrees so as to intercept rays which leave the back surfacein the region 40 at an angle of 60 degrees with respect to the normal tothe back surface and reflect theselight rays at a beam angle of 28degrees so as to fall in the opposite, outer quarter of the format. RaysW and W are typical. The facet 43, for example, may be oriented at anangle of 17 degrees so as to intercept rays which leave the more distantportion of the region 40 at the smaller angle of 45 degrees with respectto the normal to the back surface and reflect these rays at an angle of28 degrees so as to fall in the opposite, outer quarter of the format.Rays Z, and Z are typical. A pair of facets 44 and 45 are similarlyprovided in the end plate 16.

THE ILLUMINATION SYSTEM Turning now from the consideration of theillumination module 10 to a consideration of a typical illuminationsystem using more than one illumination module 10, reference is firstmade to FIG. 8 which illustrates an illumination system 46 whichcomprises a bank of six illumination modules mounted in a frame 47. Itshould be noted that two of the illumination modules (indicated by thereference numeral 10) have their axes perpendicular to the direction offlight while four of the illumination units (indicated by the referencenumeral 10'') have their axes parallel to the direction of flight. Thetwo illumination modules 10' provide weighted light distribution andformat edge definition in the fore and aft directions while the fourillumination units 10" provide weighted light distribution and formatedge definition in the port and starboard directions.

The illumination system 46 illustrated in FIG. 8 includes a ram airintake 48 and manifold 49 for distributing ram cooling air through theseveral illumination units 10' and 10" via manifold conduits 50. Thepower supply and associated flash electrical circuitry are not a part ofthe present invention and, hence, are not shown.

To complete the understanding of the present invention, a descriptionfollows of a typical application of an illumination system in accordancewith the invention.

Referring to FIG. 9, there is provided a diagrammatic perspectiveillustration of the photographic geometry of a typical illuminationsystem application. By way of example, it may be assumed that therequirement is to illuminate a photographic format 74 degrees wide inthe lateral direction and 74 degrees wide in the longitudinal directionfrom an aircraft 60 at an altitude of 500 feet with the camera opticalaxis at an angle of 20 degrees aft of the aircraft nadir N. Since thecamera axis is tilted aft, the square illumination beam will image as atrapezoid 63 on the ground as illustrated. The angle a is the lateralhalf-beam-width angle which in this example is equal to 37 degrees. Theangle 0 is the angle from the nadir N to any point P, on the groundformat F. The angle (1; is the angle from the camera axis 0 to any pointP, on the ground format. 1

Assuming that the fall-off attenuation for the lens to be used is acosgb function, the required ground irradiance in joules per squaremeter is given by the expressron:

where:

E ground irradiance at the optical axis, and

Sec compensation for the cos lens fall-off characteristic. FIG. is aperspective diagrammatic illustration of a topological plot of groundirradiance E, throughout the ground format.

The beam intensity distribution necessary to produce the required groundirradiance distribution must be determined. Ground irradiance is relatedto source intensity l by:

r slant range at any point P, on the ground format, and

H the altitude of the aircraft. Compensation must be made for the cos flslant range attenuation term, and the theoretical intensity distributionin joules per steradian becomes:

I 1,, sec 0 sec" where:

I the value of radiant intensity at the nadir.

FIG. 11 is a representation of the computed theoretical radiantintensity I at plurality of representative points in the ground formatF.

Referring now to FIGS. 12 through 16, a series of diagrammaticrepresentations of the ground format F are provided which illustrate thegeneration of the requisite beam intensity distribution for the groundformat F by a composite of beams from an array of six' illuminationmodulesf In FIG. 12, the aft portion of the: ground format F isilluminated by two illumination modules each arranged with its lamp axisparallel to the line of flight and inclined at an angle of 49.6 degreesaft of the nadir and having a beam width of 74 degrees in a planeperpendicular to the direction of flight beam width of l4.8 degrees in aplane parallel to the direction of flight. The rleative intensityweighting factor of eachmodule in the outer lateral 13 degrees of thebeam as compared to the central portion of the beam is 2 to l. The twoillumination beams are congruent on the formatF, producing the compositeintensity distribution as shown in FIG. 12.

The intensity distribution pattern shown in FIG. 13 is produced by thebeams of two illumination modules inclined 12.6 degrees aft of the nadirwith their lamp axes parallel to the direction of flight. The beam widthfrom each module in a plane perpendicular to the line of flight is 74degrees, and the beam width in a plane parallel to the line of flight is59.2 degrees. In this case, the weighting factor of each module for theouter 13 degrees as compared with the central portion of the beam is0.50 to 0.25. It should be noted that while the weighting ratio of themodules providing the illumination pattern of FIG. 13 is the same as forthe modules providing the illumination pattern of FIG. 12 (i.e., twiceas much light intensity in the outer 13 degrees of the format as in thecentral portion), the area of the beam in FIG. l3'is much larger withthe result that the overall density of illumination in the FIG. 13pattern is less than that of the FIG. 12 pattern.

The illumination pattern shown in FIG. 14 is produced by a singleillumination module arranged with its lamp axis parallel to the line offlight and inclined 38.5 degrees aft of the nadir. The dimensions of thebeam from this module are 74 degrees in the plane perpendicular to theline of flight and 37 degrees in the plane of the line of flight. Thereflective surfaces in this module are contoured to provide a weightingfactor for the outer 25 degrees on each side of the pattern comparedwith the central portion of the pattern of 1.35 to 0.45, as indicated.

The sixth module is arrangedwith its lamp axis perpendicular to thedirection of flight. The center line of the beam is directed 20 degreesaft of the nadir (coincident with the camera optical axis), and thereflector is designed to provide a 74 degree by a 74 degree beam tocover the entire area of the format as illustrated in FIG. 15. In thisinstance, the reflector is contoured to weight the fore and aft l4.8degree sectors of the pattern as compared with the central portion ofthe pattern by a factor of 1 to 0.25.

FIG. 16 provides a composite of the illumination patterns shown in FIGS.12 through 15 with an average intensity summation for each segment ofthe format F. In

FIG. 17, the approximate average intensity provided in each segment iscompared with the approximate theoretical average intensity (in blocks)for each segment from the intensity distribution map of FIG. 11. Asseen, the illumination system provides an intensity distribution verycomparable to the theoretical intensity distribution for the particularapplication.

From the foregoing description, it is apparent that the presentinvention provides a versatile optical illumination system which has theimportant capability for producing an efficient, well-definedillumination intensity distribution which will compensate for theattenuating effects of slant range and lens fall-off.

The specific examples herein shown and described are illustrated only.Various changes in structure will, no doubt, occur to those skilled inthe art, and these changes are to be understood as forming a part ofthis invention insofar as they fall within the spirit and scope of theappended claims.

What is claimed is:

1. An aircraft mounted ground illumination system for an optical lenssystem comprising:

a plurality of illumination modules, each comprising a cylindrical lightsource and a trough-like reflector having a rectangular beam apertureand a pair of curved reflective surfaces parallel with the gemoetricaxis of said cylindrical light source and formed to provide reflectedlight distribution weighted in a plane perpendicular to the geometricaxis of said light source, at least a first one of said illuminationmodules being arranged with its axis parallel to a given direction so asto provide light distribution weighted in a plane perpendicular to saidgiven direction, and at least a second one of said illumination modulesbeing arranged with its axis perpendicular to said given direction so asto provide light distribution weighted in a plane parallel to said givendirection, at least said first and second illumination modulesilluminating in common at least a predetermined portion of a desiredground-imaged format additively in accordance with the expression wherel is radiant light intensity, I is the value of radiant intensity at thenadir, 0 is the slant range angle to said portion as measured from thenadir, (b is the optical lens fall-off angle to said portion as measuredfrom the optical axis of said desired format, and n is an exponentindicative of the off-axis optical characteristic of the optical lensused.

2. An optical illumination system as defined in claim 1 wherein each ofsaid illumination modules has at least one curved reflective surfacewhich is parallel to the axis of said cylindrical light source and whichextends through a series of points, said reflective surface at each ofsaid points being spaced from the geometric axis of said cylindricallight source and having a slope one another about a median plane definedby the geometric axis of said cylindrical light source and the centerline of the illumination beam aperture of said reflector, each of saidreflective surfaces extending through a series of points, the reflectivesurface at each of said points being spaced from the geometric axis ofsaid cylindrical light source and having a slope reflecting thetangentially-emitted incident ray nearer the aperture of the reflectorat a predetermined half-beam-width angle relative to said median planefor incidence 'upon the opposite outermost edge of the illuminationformat, the spacing of said points from the geometric axis of saidcylindrical light source being increased at each successive one of saidpoints to diminish the divergence angle of the reflected rays at eachsuccessive one of said points in said series from a predeterminedmaximum angle at the first point in said series to a predeterminedminimum angle at the last point in said series.

4. An optical illumination system as defined in claim 1 furthercomprising:

an intake for cooling air; manifold means for distributing cooling airfrom said intake to said illumination modules, each of said illuminationmodules having an optically transparent quartz tube within which saidcylindrical illumination source is coaxially disposed; means forconducting cooling air from said manifold means to one end of saidquartz tube; and means for exhausting cooling air from the opposite endof said quartz tube. 5. An optical illumination system as defined inclaim 4 wherein each of said illumination modules includes electricalconnection clips for the-electrodes of said cylindrical light source,said connection clips having a plurality of radial fins interposed inthe cooling air flow through said quartz tube.

6. An optical illumination unit comprising: a cylindrical light source,and a trough-like reflector having a rectangular beam aperture andhaving at least one curved reflective surface which is parallel to theaxis of said cylindrical light source and which extends through a seriesof points; g the distance L, of said reflective surface from thegeometric axis of said cylindrical light source at the first(rearwardmost) point in said series being defined by the expressionwherein R is the radius of said cylindrical light source, J, is thepredetermined half-beam-width angle, and K, is a selected angle which isless than a half-beam-width angle;

the slope Y, of said reflective surface at said first point in saidseries being defined by the expression wherein X, is the polarcoordinate angle of said first point P, in said series as related to thegeometric axis of said cylinder light source as measured from a centerplane passed through the geometric axis of the light source and thecenter line of the illumination beam aperture of said reflector;

the slope Y, of said reflective surface at every other point P, in saidseries being defined by the espression wherein J, is constant equal tothe half-beam-width angle 1,, K,, is an angle selected to have thecharacteristic K,, K,, l K,, and X is the polar coordinate angle of thepoint P, with X having the characteristic X,

ric axis of said cylindrical light source and formed to providereflected light distribution weighted in a plane perpendicular to thegeometric axis of said light source, at least a first one of saidillumination modules being arranged with its axis parallel to a givendirection so as to provide light distribution weighted in a planeperpendicular to said given direction, and at least a second one of saidillumination modules being'arranged with its axis perpendicular to saidgiven direction so as to provide light distribution weighted in a planeparallel to said given direction, at least said first and secondillumination modules illuminating in common at least a predeterminedportion of a desired ground-imaged format additively in accordance withthe expression I E H /Cos 6 where I is radiant light intensity, E isground irradiance compensated for the off-axis optical characteristic ofthe optical lens used, H is the aircraft altitude at which the opticallens is to be used, and 0 is the slant range angle to said portion asmeasured from the nadir.

1. An aircraft mounted ground illumination system for an optical lenssystem comprising: a plurality of illumination modules, each comprisinga cylindrical light source and a trough-like reflector having arectangular beam aperture and a pair of curved reflective surfacesparallel with the geometric axis of said cylindrical light source andformed to provide reflected light distribution weighted in a planeperpendicular to the geometric axis of said light source, at least afirst one of said illumination modules being arranged with its axisparallel to a given direction so as to provide light distributionweighted in a plane perpendicular to said given direction, and at leasta second one of said illumination modules being arranged with its axisperpendicular to said given direction so as to provide lightdistribution weighted in a plane parallel to said given direction, atleast said first and second illumination modules illuminating in commonat least a predetermined portion of a desired ground-imaged formatadditively in accordance with the expression I Io sec2 theta secn phiwhere I is radiant light intensity, Io is the value of radiant intensityat the nadir, theta is the slant range angle to said portion as measuredfrom the nadir, phi is the optical lens fall-off angle to said portionas measured from the optical axis of said desired format, and n is anexponent indicative of the off-axis optical characteristic of theoptical lens used.
 2. An optical illumination system as defined in claim1 wherein each of said illumination modules has at least one curvedreflective surface which is parallel to the axis of said cylindricallight source and which extends through a series of points, saidreflective surface at each of said points being spaced from thegeometric axis of said cylindrical light source and having a slopereflecting the tangentially-emitted incident ray nearer the aperture ofthe reflector at a predetermined half-beam-width angle relative to acenter plane defined by the geometric axis of the light source and thecenter line of illumInation beam aperture of said reflector forincidence upon the opposite outermost edge of the illumination format,the spacing of said points from the geometric axis of said cylindricallight source being increased at each successive one of said points todiminish the divergence angle of the reflected rays at each successiveone of said points in said series from a predetermined maximum angle atthe first point in said series to a predetermined minimum angle at thelast point in said series.
 3. An optical illumination system as definedin claim 5 wherein each of said illumination modules has a pair ofcurved reflective surface regions symmetrical with one another about amedian plane defined by the geometric axis of said cylindrical lightsource and the center line of the illumination beam aperture of saidreflector, each of said reflective surfaces extending through a seriesof points, the reflective surface at each of said points being spacedfrom the geometric axis of said cylindrical light source and having aslope reflecting the tangentially-emitted incident ray nearer theaperture of the reflector at a predetermined half-beam-width anglerelative to said median plane for incidence upon the opposite outermostedge of the illumination format, the spacing of said points from thegeometric axis of said cylindrical light source being increased at eachsuccessive one of said points to diminish the divergence angle of thereflected rays at each successive one of said points in said series froma predetermined maximum angle at the first point in said series to apredetermined minimum angle at the last point in said series.
 4. Anoptical illumination system as defined in claim 1 further comprising: anintake for cooling air; manifold means for distributing cooling air fromsaid intake to said illumination modules, each of said illuminationmodules having an optically transparent quartz tube within which saidcylindrical illumination source is coaxially disposed; means forconducting cooling air from said manifold means to one end of saidquartz tube; and means for exhausting cooling air from the opposite endof said quartz tube.
 5. An optical illumination system as defined inclaim 4 wherein each of said illumination modules includes electricalconnection clips for the electrodes of said cylindrical light source,said connection clips having a plurality of radial fins interposed inthe cooling air flow through said quartz tube.
 6. An opticalillumination unit comprising: a cylindrical light source, and atrough-like reflector having a rectangular beam aperture and having atleast one curved reflective surface which is parallel to the axis ofsaid cylindrical light source and which extends through a series ofpoints; the distance Ls of said reflective surface from the geometricaxis of said cylindrical light source at the first (rearwardmost) pointin said series being defined by the expression Ls R/arctan ((Js - Ks)/2)wherein R is the radius of said cylindrical light source, Js is thepredetermined half-beam-width angle, and Ks is a selected angle which isless than a half-beam-width angle; the slope Ys of said reflectivesurface at said first point in said series being defined by theexpression Ys (Js + Ks +2Xs)/4 wherein Xs is the polar coordinate angleof said first point Ps in said series as related to the geometric axisof said cylindrical light source and as measured from a center planepassed through the geometric axis of the light source and the centerline of the illumination beam aperture of said reflector; the slope Ynof said reflective surface at every other point Pn in said series beingdefined by the expression Yn (Jn + Kn + 2Xn)/4 wherein Jn iS constantequal to the half-beam-width angle Js, Kn is an angle selected to havethe characteristic Kn>Kn 1 >Ks, and Xn is the polar coordinate angle ofthe point Pn with Xn having the characteristic Xn Delta X + Xn 1; thedistance Ln of said reflective surface from the geometric axis of saidcylindrical light source at Pn being defined by the expression Ln Ln 1(1 + sin Delta X tan (Xn 1 + ( Delta X/2) - Yn 1)).
 7. An aircraftmounted ground illumination system for an optical lens systemcomprising: a plurality of illumination modules, each comprising acylindrical light source and a trough-like reflector having arectangular beam aperture and a pair of curved reflective surfacesparallel with the geometric axis of said cylindrical light source andformed to provide reflected light distribution weighted in a planeperpendicular to the geometric axis of said light source, at least afirst one of said illumination modules being arranged with its axisparallel to a given direction so as to provide light distributionweighted in a plane perpendicular to said given direction, and at leasta second one of said illumination modules being arranged with its axisperpendicular to said given direction so as to provide lightdistribution weighted in a plane parallel to said given direction, atleast said first and second illumination modules illuminating in commonat least a predetermined portion of a desired ground-imaged formatadditively in accordance with the expression I EgH2/Cos2 theta where Iis radiant light intensity, Eg is ground irradiance compensated for theoff-axis optical characteristic of the optical lens used, H is theaircraft altitude at which the optical lens is to be used, and theta isthe slant range angle to said portion as measured from the nadir.